I learned very quickly from playing with colors that some are notorious for staining the canvas. When I make a mistake, I can correct it by warming the wax with my hairdryer and lifting away with a paper towel. With certain colors, such as blue, dark red, bright yellow and black, a shadow of that color is permanently stained into the canvas. Because wax is so translucent, those stains will always show through.
One day I wanted to make a sky scene with a moon. Initially I made a barely visable pencil outline of my moon and applied a thick layer of clear wax over and around it so that if any of my blue flowed into it, I could quickly push it away before the area stained. Once the first layer of blue was applied to my canvas, I used my hairdryer to push the color away from the moon to create the appearance of the color fading into the light. In doing this, the moon seemed to loose it's shape and was a blob of white in the sky. I had to go back with a brush and lightly outline the moon with the palest shade of blue I could make in order to redefine it's shape. This outline destroyed the illusion of the sky fading into the light, and that really bothered me.
Then it occured to me that I could use the staining problem to define the shape of my moon. With a new dry canvas, I drew another moon and then surrounded it with a strong blue wax. I filled my moon with clear wax and used the hair dryer to push all the blue away. The only color remaining close to the moon was the pale stain on the canvas. I was very pleased with the results.
Knowing that my reds and yellows will create the same staining effect, I used them for a sunny sky.